ECOLOGY AND POPULATION BIOLOGY Linking Parasitic Plant-Induced Host Morphology to Tritrophic Interactions

We investigated the tritrophic interactions among southwestern dwarf mistletoe [Arceuthobium vaginatum (Willd.) Presl subsp. cryptopodum], mistletoe herbivores, and host pine (Pinus ponderosaDougl. ex Laws. and C. Laws. variety scopulorum Engelm.)-associated predators. In an observational study, we characterized differences in pine-associated arthropods and pine branch morphology between branches either parasitized by mistletoe (brooms) or not visibly infected. Compared with noninfected branches, brooms had a more reticulate branching structure, collected 36 times more dead needles and supported 1.7 times more arthropod predators. In a manipulative Þeld experiment, we investigated whether pine-associated predators fed upon lepidopteran herbivores of mistletoe and thereby reduced herbivore damage to the parasite. Over a 30-d trial, herbivores fed upon approximately two-thirds of available mistletoe shoots. Predator removal increased herbivore survival by 56%but hadnodetectable effect on the level of herbivory damage.We speculate that herbivores compete formistletoeshootsandthat increasedper-capita feedingcompensatedforpredatorreductionofherbivore abundance. In summary, our results demonstrate that mistletoe parasitism altered the pine arthropod community, including an increase in the density of predators that likely feed upon mistletoe herbivores.